This invention relates to improvements in steering column assemblies. It in particular relates to steering columns which form part of an electrically power assisted steering system.
A steering assembly typically comprises a steering shaft which connects a steering wheel to a pair of road wheels. A portion of the shaft is located within the driving compartment of the vehicle and is supported within a steering column by one or more bearings. The steering column and the shaft are usually designed so that they will collapse in the event of an accident in which the weight of a driver is applied to the steering wheel. This allows the wheel to move away from the driver, reducing the loading on the driver The column is typically made in two tubular sections which collapse by one part sliding over the other.
With the introduction of electrically assisted power steering systems in which an electric motor is used to apply an assistance torque to the steering shaft, it is often found that a relatively high proportion of the steering shafts length is taken up by the motor or its associated components such as a gearbox and its housing. This reduces the length of steering column that can be permitted to collapse. Most vehicle manufacturers today specify that the minimum acceptable collapse is 100 mm, yet also place limitations on the total length of the steering column of between 480 and 580 mm.
In addition to the requirement for collapse a steering column must also be able to pass an offset crash test set out in regulatory procedures. To avoid excess friction between the two overlapping parts a minimum overlap should be provided. If too short the tubes may bind. Most manufacturers again specify a minimum overlap to enable the column to pass such a test.
Using a steering column with two tubes that has a minimum overlap of 100 nm and a collapse of 100 mm requires 300 mm of the overall shaft length to be committed to the collapse function. For a steering column shaft at the shorter end of a manufacturers range this may only leave a small amount of room for the EPAS device and the steering wheel. In many cases there is not enough room to meet these requirements.
It is therefore an object of the invention to provide a steering column assembly which provides for a relatively high collapse travel from a relatively short column length.